Execution engine
Specification
- list of fragments for the queries
- S=(S1, S2) join with R=(R1, R2, R3)
- execution subqueries out-of-order
- multiple stages when precedence is required
- execute multiple subqueries together
- if S1, S2 and R3 are on cache, (S1, R3) and (S2, R3) has to happen together